Transmission and distribution lines often include solenoid actuated high-voltage switches and circuit breakers that are opened and closed in response to a remotely supplied signal, for example, a signal supplied from a system control center or substation control panel. Each time that a switch or circuit breaker opens or closes, the contacts within it may be subjected to deterioration due to arcing, particularly if the line current is interrupted at its peak or if the device is closed at the peak of the periodically varying voltage. Arcing can also produce radio frequency interference (RFI). More importantly, each time that a switch or circuit breaker opens or closes at a current or voltage peak, respectively, damaging transients may be generated on the line by the resulting arcing or prestrikes. For example, if the current in a line connected to a capacitor bank or to a capacitive load is switched, the voltage on the bus may momentarily collapse to zero and then begin to oscillate at high frequencies and at high magnitudes. Such transients can damage equipment connected to the line and are very undesirable.
Conventional switches and circuit breakers are not designed to open or close at times appropriate to minimize stress and arcing. Instead, once a switching command is issued, the devices begin to open or close immediately as current flows through their solenoid actuation circuits. By monitoring the voltage and current on a bus, it would be possible to delay enabling the current to the solenoid that actuates a switch or circuit breaker for an appropriate time interval so that the device actually opens when the current waveform is crossing zero and closes when the voltage waveform is crossing zero. The delay introduced in enabling the electrical current to the solenoid or other actuator of the switch or circuit breaker should therefore include the response time of the device in opening or closing, i.e., an appropriate time for the device to react after its actuator is energized to open or close the switch or breaker contacts. However, the response time of the operating mechanism in the switch or circuit breaker typically changes with use and over time. For example, the force developed by springs used in the operating mechanism tend to change with age and usage, and because of the influence of ambient environmental conditions, such as temperature, barometric pressure, and humidity. Thus, it is not practical to simply measure the response time of a switch or circuit breaker at the time of its manufacture to determine the timing of a switching operation, because after the device has been in operation for several years, its response time will have changed substantially.
The advantages of closing a circuit breaker when the voltage on the line crosses zero and opening the breaker when the current is zero are discussed in a paper entitled, "Switching to Lower Transients," by R. Avinsson and C. Solver, ABB HV Switchgear Corporation, Ludvika, Sweden (March 1991). To reduce transient disturbances caused by operating a circuit breaker to connect a capacitor bank to a 130 KV line used by a Swedish utility, a microprocessor-based device was developed to open and close the circuit breaker when the current and voltage on the line were such as to likely minimize transients. Since long term variations in the circuit breaker closing time were expected, the control device was designed to self adjust the closing and opening times to compensate for such changes. While enabling details are omitted from the paper, it appears that the microprocessor in this device compares the predicted closing (or opening) time with the actual closing (or opening) time and adjusts the predicted time next used to operate the circuit breaker by applying one-half of the measured error. The predicted time used in controlling the circuit breaker is referenced to either the voltage or current on the line. This approach adaptively controls the circuit breaker based on errors in the predicted closure time of the breaker for a purely reactive load, within an error range of .+-.1 ms; yet, it does not specifically detect transients caused by operation of the breaker and adaptively control the circuit breaker to eliminate such transients when the breaker is next operated. Other sources of delay in the onset or interruption of current flow through the circuit breaker that might give rise to transients or restrikes, such as environmental conditions, are thus not compensated by the ABB HV Switchgear Corp. circuit breaker control. Furthermore, the device does not seem capable of compensating a breaker when the phase angle between current and voltage on the line is not nearly ninety degrees, i.e., for other than a purely reactive load.
Clearly, a switch controller that compensates for changes in the response time of a switch or circuit breaker operating mechanism under all conditions of operation is desirable. The controller should be able to adapt to changes in the response time of the switching device caused by aging, for virtually any phase angle associated with a load, so that operation of the switching device is initiated at an appropriate time selected to ensure that current flow on the bus is actually enabled and interrupted by the device at near zero voltage and near zero current crossings, respectively, to substantially eliminate switching transients in subsequent switching operations. Further, the controller should compensate for ambient environmental conditions in determining the appropriate times at which to initiate switching operations without producing transients.